touch: Change long-press propagation phase

The GtkGestureLongPress that's responsible for opening up the background context menu was not receiving any motion events, presumably due to the GtkGridView catching them in the capture-phase already, and was thus unable to detect passing the touch-drag threshold. To re-enable items to be long-pressed as well, the background long-press callback checks if the pointer is above an item beforehand.

Fixes #2872 (closed)